To seal rotating shafts of pumps, compressors and the like, it is known to provide a non-contacting shaft seal on the shaft, which includes an axially adjacent pair of seal rings wherein one seal ring rotates with the shaft and the other seal ring is non-rotatably connected to a seal housing. The seal rings each include an end face which faces axially wherein the seal faces are disposed in close opposing relation to define a sealing region extending radially between the outer and inner diameters of the seal rings. The fluid being sealed in conventional seals can either be a liquid or a gas, and the sealing region prevents or minimizes migration or leakage of the fluid radially across the seal faces.
More particularly, the seal faces typically are disposed in contact with each other when the shaft is not rotating to thereby define a static seal. Further, at least one of the seal faces includes a hydrodynamic face pattern that generates a fluid film between the seal faces during shaft rotation to reduce if not eliminate contact between the seal faces.
Hydrodynamic face patterns are known and include wavy faces, spiral grooves, T-grooves and the like. The face patterns are formed in the seal faces through various processes and typically involve providing a flat face and then removing material from the seal face to a very small depth. Typically the seal rings comprise a carbon seal ring and a silicon carbide seal ring which is a well known material combination.
While such mechanical seals are readily useable in a wide variety of environments, in specific sealing applications, for example, for the power industry and primarily for boiler feed seals in water, a particular electro-corrosion phenomenon occurs that results in damage to the seal faces. This damage manifests itself in different forms, and for example, may manifest itself as pitting on the outer diameter of the silicon carbide (SiC) seal face. The corrosive phenomenon also may manifest itself on the carbon seal interface.
It is believed that this electro-corrosion is the result of a form of static electrical charge that develops, possibly by friction between the relatively rotating running seal faces, which charge develops on one or both of the seal faces and then discharges to the mating face or another nearby metal component of the seal assembly or else to the surrounding water. This discharge is believed to result in damage to the seal rings and premature wear thereof.
With respect to the sealing applications in which such phenomenon occurs, one such environment is the aforementioned boiler feed seals wherein such boiler feed seals may use ultra pure water having a low conductivity wherein such water also may include an oxygen treatment therewith. This type of low conductivity water or other similar fluid being sealed is believed to facilitate the occurrence of the electro-corrosion phenomenon.
To date attempts have been made to solve this problem in different ways. For example, one such attempt to resolve this problem has been the use of similar materials for the seal faces, specifically, silicon carbide against silicon carbide. Normally, conventional seals as mentioned above comprise carbon and silicon carbide seal rings wherein one theory is that the use of the different seal face materials is one factor in the occurrence of the electro-corrosion thereof. However, the use of silicon carbide against silicon carbide results in a seal ring combination that is undesirable in that such seal rings are sensitive to damage when contact of the seal faces occurs which can happen during starts and stops of the shaft as well as operational upsets that might occur during normal operation of a pump which upsets may force the seal faces together and result in damage to the seal faces. Further, it has been found that even silicon carbide versus silicon carbide can result in problems if these seal faces have a high ratio of electrical conductivity wherein surface damage can still occur.
Also, U.S. Pat. No. 4,391,450 addresses the problem of electro-kinetic corrosion occurring in the mating surfaces of the seal rings of a mechanical seal used in sea water wherein the mechanical seal is used in the presence of an electrolytic liquid, i.e. sea water. The '450 patent proposes a solution for such electro-kinetic corrosion which solution is allegedly achieved by selecting the materials for both of the seal rings wherein both seal rings are selected so to both be electrically insulative or non-conductive so as to not support an electro-chemical reaction either at all or after a brief initial stage.
It also has been known to employ grounding devices on a rotatable shaft to attempt to draw away stray currents that develop. This has had some success but not in every case. This does result in additional expense, and may be limited by availability of space, and the improper application of such a device can possibly generate more charge and damage to other components in the pump/bearing/seal system.
It is therefore an object of the invention to provide a mechanical seal and a manufacturing process for the mechanical seal so as to result in a mechanical seal which greatly resists if not eliminates the occurrence of such electro-corrosion.
To achieve these objectives, the invention generally relates to a mechanical seal having a pair of opposed seal rings which comprise a conventional combination of a silicon carbide seal ring and a carbon seal ring having opposing seal faces which define the sealing region therebetween.
In particular, the invention relates to the structure of such seal rings and the process for forming the seal rings which involves the application of a laser surface treatment which preferably is applied to the silicon carbide face so as to modify the surface properties thereof. By using a particular laser wavelength and energy density, it is possible to change the electrical conductivity of the silicon carbide face by the application of the laser to the entire seal face and thereby alter the surface properties of the silicon carbide from a normal insulative type condition to a condition where the silicon carbide seal face material acts as a conductor. The modified conductivity value of the seal face results from the seal face treatment and the conductivity value is set relative to the conductivity value of the opposing carbide seal face to relative values that results in a significant reduction or complete elimination of electro-corrosion.
Where micro-topography seal face features are provided in the seal ring, a laser machining process is known from published U.S. Patent Application No. 2003/0209859 A1 which is owned by the assignee of the present invention and incorporated herein in its entirety by reference. The laser machining process disclosed in this published application involves laser micro-machining of the seal face material at an energy level which results in ablation or removal of the seal face material to a selected depth and in a pattern which defines a desired pattern.
The aforementioned laser treatment process preferably is performed after the laser micromachining process at a lower energy level which avoids or substantially minimizes surface ablation. As such, the seal face maintains a flat, lapped surface condition in those areas outside the microtopography face features and maintains the desired shape of such face features. However, the energy level of the laser treatment process is set at a lower level which avoids or substantially eliminates ablation so that the treated seal face has the conductive properties thereof modified as desired to address the electro-corrosion problem.
Other objects and purposes of the invention, and variations thereof, will be apparent upon reading the following specification and inspecting the accompanying drawings.
Certain terminology will be used in the following description for convenience in reference only, and will not be limiting. For example, the words “upwardly”, “downwardly”, “rightwardly” and “leftwardly” will refer to directions in the drawings to which reference is made. The words “inwardly” and “outwardly” will refer to directions toward and away from, respectively, the geometric center of the system and designated parts thereof. Said terminology will include the words specifically mentioned, derivatives thereof, and words of similar import.